1. Field of the Invention
This invention relates to transducers and in particular to strain gage transducers wherein different temperature conditions may adversely affect operation of the apparatus.
2. Description of the Prior Art
In one form of strain gage transducer, a plurality of strain gages are mounted on the surface of a supporting body, illustratively comprising a beam, for producing electrical signals corresponding to strains induced in the supporting body. Such strain gage transducers may be made extremely accurate. However, it has been found that the accuracy thereof may be adversely affected by temperature differentials occurring in the apparatus and it has been conventional to provide in such apparatus temperature compensating means in order to minimize such adverse affect.
More specifically, an illustrative strain gage transducer, wherein strain gages are mounted on surface portions of a beam, is illustrated in U.S. Pat. No. 4,107,626 of David A. Kiewit, which patent is owned by the assignee hereof. The signal developed therein from the piezoelectric strain gage transducers utilize surface acoustic wave paths extending through the tension and compression portions of the beam surface. As disclosed in the Kiewit patent, the transducers may be formed by deposition using thin film techniques.
An early form of such strain gage is illustrated in U.S. Pat. No. 2,350,972 of Arthur C. Ruge, which teaches the use of materials having positive and negative thermal coefficients of resistance in forming the strain gage elements so as to provide an overall fixable temperature coefficient at any desired value between the two thermal coefficients. As shown in Ruge, the strain gage is generally symmetrical.
Herman T. Gay shows, in U.S. Pat. No. 3,184,962, a strain-type transducer which utilizes a single element resistant gage so that the transducer is compensated for changes in different parameters produced by changes in the ambient temperature. Gay points out that where two resistance elements are used in a resistant strain gage, one solution to the temperature differential problem is to cause the first resistance element to have a modulus of elasticity which increases with an increase in temperature and the other element to have a modulus of elasticity which decreases in the same proportion with an increase in temperature. Gay teaches that the characteristics of the strain sensitive material be combined with the characteristics of the strain material so as to produce substantially zero change in sensitivity and electrical resistance of the responsive material due to changes in temperature.
In U.S. Pat. No. 4,116,075 of Werner Ort, a transducer is disclosed wherein the strain gages are mounted either on a diaphragm or flexure spring mechanically coupled to a diaphragm. The diaphragm is mounted on a clamping ring and the strain gages are disposed at different distances from the clamping ring. Ort teaches that heat transfer from the different strain gages may be equalized relative to the clamping ring by varying the areas of the strain gages, the spacing of the conductors of the strain gages, the relative positioning of the strain gages, or the width of portions of the flexure spring. Alternatively, Ort suggests that slots may be provided adjacent the strain gages in the flexure spring and illustrates the use of such slots in a symmetrical arrangement in FIG. 6 of the patent. Alternatively, Ort teaches the use of a flexure spring having a pair of legs connecting a centerpiece to the clamping ring, with the structure again being raised in a symmetrical configuration, as shown in FIG. 7 of the patent.